The present invention relates to internal combustion engine diagnostics and more specifically to apparatuses and methods for determining the spatial and temporal nonuniformities of CO2 in an intake fluid stream.
Internal combustion engines typically suffer from the ability to produce undesirable NOX emissions. Experience has revealed that more NOX emissions are formed at higher combustion temperatures and that NOX formation has a nonlinear dependence on temperature. More specifically, lowering the combustion temperature a little can result in relatively large reductions in NOX formation.
Exhaust-gas recirculation, EGR, is a technology used to reduce automotive NOX emissions, and which involves mixing some of the engine exhaust with the intake air. The exhaust gas acts as a diluent in the inlet air that reduces peak combustion temperature. Ideally, the air/exhaust mixture, or EGR fraction, is uniform across the various cylinders of a multi-cylinder engine. However, practically the EGR fraction can vary from cylinder to cylinder and cycle to cycle due to various spatial and temporal nonuniformities; e.g., non-ideal mixing, intake-manifold restrictions, and overlap of valve events with manifold resonating. Such nonuniformities can cause one cylinder to reach a limit (e.g., incomplete combustion, etc.) earlier than the other cylinders, and can limit the performance of the other cylinders. Ultimately, the result is lost efficiency and increased engine emissions.
An EGR probe that can be used to identify non-uniformities, track their origins and assess mitigation strategies could be a powerful tool for optimizing efficiency and performance of multi-cylinder engines. For example, a probe of this nature may be capable of identifying spatial or temporal fluctuations in the performance of the EGR system, which may result from the design or configuration of the EGR system, the intake manifold, engine events or other factors.
In the past, EGR probes that rely on capillary action have been developed to assist in mapping CO2 variations within an engine intake manifold. These tiny capillary probes allow samples to be extracted from the intake manifold and delivered to remote equipment for analysis. The capillary probes are capable of being spatially translated so that they can take samples from different location within the exhaust manifold. For example, the capillary probes are capable of being mounted in different apertures in the intake manifold and of being inserted to different depths within a given aperture. The samples extracted using capillary probes are analyzed remotely using absorption spectroscopy or mass spectrometry, or other analytical technique, to determine CO2 concentration. Although a meaningful advance, conventional capillary probes suffer from a variety of disadvantages. Perhaps most notably, capillary-probe-based diagnostic systems are not fast enough to measure fast valve-time scale, crank-angle resolved variations. As a result, the use of capillary probes can place significant limitations on the capabilities the diagnostic system.
Another technique previously used to measure EGR fraction variations by cylinder is the use of oxygen sensors. Exhaust oxygen sensors are common on engines for vehicles and aid the engine system in controlling the air-to-fuel ratio during combustion. They function based on a solid state electrochemical cell (normally composed of a metal oxide). For the application of measuring EGR fraction distribution, they have limitations related to diffusion, temperature, and pressure. Gas measurement required diffusion into the electrochemical cell and also through a protective porous housing (commonly ceramic based); the time required for the diffusion process can limit temporal response especially relative to rapid cylinder-to-cylinder and cycle-to-cycle time scales. Also, the oxygen sensors must be heated to work effectively, and cool intake gas temperatures pose problems for the sensors to maintain the necessary sensor temperature. Lastly, variations in pressure in the intake system (often occurring especially for boosted engines) can alter oxygen sensor measurements.